Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -201,6 +201,15 @@
   set(CLANG_DEFAULT_CXX_STDLIB "")
 endif()
 
+set(CLANG_DEFAULT_RTLIB "" CACHE STRING
+  "Default runtime library to use (libgcc or compiler-rt)")
+if (NOT(CLANG_DEFAULT_RTLIB STREQUAL "" OR
+        CLANG_DEFAULT_RTLIB STREQUAL "libgcc" OR
+        CLANG_DEFAULT_RTLIB STREQUAL "compiler-rt"))
+  message(WARNING "Resetting default rtlib to use platform default")
+  set(CLANG_DEFAULT_RTLIB "")
+endif()
+
 set(CLANG_DEFAULT_OPENMP_RUNTIME "libomp" CACHE STRING
   "Default OpenMP runtime used by -fopenmp.")
 
Index: include/clang/Config/config.h.cmake
===================================================================
--- include/clang/Config/config.h.cmake
+++ include/clang/Config/config.h.cmake
@@ -11,6 +11,9 @@
 /* Default C++ stdlib to use. */
 #define CLANG_DEFAULT_CXX_STDLIB "${CLANG_DEFAULT_CXX_STDLIB}"
 
+/* Default runtime library to use. */
+#define CLANG_DEFAULT_RTLIB "${CLANG_DEFAULT_RTLIB}"
+
 /* Default OpenMP runtime used by -fopenmp. */
 #define CLANG_DEFAULT_OPENMP_RUNTIME "${CLANG_DEFAULT_OPENMP_RUNTIME}"
 
Index: lib/Driver/ToolChain.cpp
===================================================================
--- lib/Driver/ToolChain.cpp
+++ lib/Driver/ToolChain.cpp
@@ -526,16 +526,16 @@
 
 ToolChain::RuntimeLibType ToolChain::GetRuntimeLibType(
     const ArgList &Args) const {
-  if (Arg *A = Args.getLastArg(options::OPT_rtlib_EQ)) {
-    StringRef Value = A->getValue();
-    if (Value == "compiler-rt")
-      return ToolChain::RLT_CompilerRT;
-    if (Value == "libgcc")
-      return ToolChain::RLT_Libgcc;
-    getDriver().Diag(diag::err_drv_invalid_rtlib_name)
-      << A->getAsString(Args);
-  }
+  const Arg* A = Args.getLastArg(options::OPT_rtlib_EQ);
+  StringRef LibName = A ? A->getValue() : CLANG_DEFAULT_RTLIB;
 
+  if (LibName == "compiler-rt")
+    return ToolChain::RLT_CompilerRT;
+  if (LibName == "libgcc")
+    return ToolChain::RLT_Libgcc;
+  if (A)
+    getDriver().Diag(diag::err_drv_invalid_rtlib_name) << A->getAsString(Args);
+
   return GetDefaultRuntimeLibType();
 }